Luminous burner

ABSTRACT

An apparatus and method for creating a flame-on-water effect proves a burner connected to a gas supply. The burner includes a plurality of apertures disposed vertically lower than a gas supply opening within the burner. As such, when the burner is submerged below the surface of a body of water, a gas pocket is maintained within the burner above the apertures. During operation, the burner provides a dispersed supply of gas under water, which gas rises to the surface and is ignited to create the flame-on-water effect.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser. No. 60/544,799, which was filed on Feb. 13, 2004, the entirety of which is hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention is in the field of gas appliances, and more particularly in the field of gas appliances for creating a flame-on-water effect.

2. Description of the Related Art

It is common to use water features such as pools, fountains and the like to decorate structures, including homes and commercial buildings. These features enhance the appearance of such facilities and can create a more interesting and enjoyable environment. It is also common to use lighting effects, such as colored lights, to enhance the visual effect of such features.

Such water features may be enhanced by combining them with flame effects. The combination of fire and water has proven to be especially interesting and inviting to observers. The pleasing nature of such flame and water effects is enhanced when the flame source, such as a gas fixture, is not visible to the observer. However, conventional burners are unsuitable for use under water. Further, water presents challenges to properly providing and regulating a gas supply. For example, due to water disturbance of gas flow, flame-outs may be difficult to avoid. Further, an underwater gas appliance is obviously susceptible to invasion of water into the gas supply lines.

SUMMARY OF THE INVENTION

Accordingly, Applicant has noted that there is a need for a submerged gas fixture that reliably produces a flame-on-water feature, and overcomes challenges associated with supplying gas underwater.

In accordance with one embodiment, a device for delivering unignited gas below a surface of a body of water is provided. The device includes a hollow burner head having an upper surface and a lower surface. A plurality of apertures are formed through the burner member below the upper surface. A gas supply tube of the device has an opening within the head. The opening is disposed generally above the level of the apertures. In another embodiment, all the apertures are generally disposed within an aperture plane. In still another embodiment, the aperture plane is substantially horizontal.

In still another embodiment, the device includes a control apparatus communicating with the gas supply tube. The control apparatus comprises a water purge device configured to accumulate and selectively purge water within the gas supply tube.

In accordance with another embodiment, a method is provided for supplying unignited gas to a surface of a body of water. The method includes providing a hollow burner having an upper surface, a lower surface, and a plurality of apertures formed through the burner below the upper surface. A gas supply tube is provided communicating with the burner. The gas supply tube opens into the burner at a point above the plurality of apertures. The method further includes arranging the burner within a body of water so that the apertures are disposed no more than about three inches from a surface of the body of water.

For purposes of summarizing the invention, certain embodiments, advantages, and features have been described herein. Of course, it is to be understood that not necessarily all such embodiments, advantages, or features are required in any particular embodiment. Additionally, it is to be understood that the above summary is not intended to limit in any way the embodiments, advantages, or features described below in the Detailed Description of Preferred Embodiments or the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an embodiment of an apparatus for producing a flame-on-water effect in operation.

FIG. 2 is a partially cutaway side view of a portion of the embodiment of FIG. 1.

FIG. 3 is a top view of one embodiment of a luminous burner head.

FIG. 4 is a bottom view of the burner head of FIG. 3.

FIG. 5 is a side view of the burner head of FIG. 3.

FIG. 6 is a partial cross-sectional view of a center portion of the burner head of FIG. 3, taken along lines 6-6.

FIG. 7 is a schematic diagram of an embodiment of an apparatus for producing a flame-on-water effect, and shows a water purge system.

FIG. 8 is a schematic diagram of still another embodiment of an apparatus for producing a flame-on-water effect, and shows a water purge system.

FIG. 9 is a bottom view of another embodiment of a luminous burner head.

FIG. 10 is a side view of the burner head of FIG. 9.

FIG. 11 is a schematic cut-away view of a center portion of another embodiment of a luminous burner head, showing an attachment throat entering the burner head.

FIG. 12 is a cut-away view of yet another embodiment of a luminous burner head showing a throat entering the head.

FIG. 13 is a cut-away view of still another embodiment of a luminous burner head showing a gas supply line entering the head.

FIG. 14 is a top view of a still further embodiment of a luminous burner head.

FIG. 15 is a cut-away view of a portion of the burner head of FIG. 14 taken along line 15-15.

FIG. 16 is a top view of yet another embodiment of a luminous burner head.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference first to FIG. 1, an apparatus 30 is provided for creating a flame-on-water effect. The illustrated embodiment includes a water fountain feature 32 including a fountain pool 34 that is arranged and configured to pour water into an adjacent second pool 36 (such as a swimming pool). With reference also to FIG. 2, a luminous burner 40 delivers unignited gas 42 below a surface S of the water in the first pool 34. The gas 42 rises to the water surface S and, when ignited, produces the effect of flame F on the water, as illustrated. Preferably, the gas 42 is ignited using a long match, lighter, sparkler, or the like. The illustrated luminous burner 40 comprises a burner head 44 that is attached to a gas supply line 46. The burner head 44 is mounted below the water level S so that it is not directly visible above the water. Further, the burner head 44 distributes gas over a selected area under water so that when the gas 42 reaches the surface S and is ignited, a broadly dispersed flame effect F is produced.

With reference specifically to FIGS. 3-5, a preferred embodiment of a luminous burner head 50 is presented. The burner head 50 comprises a throat 52 that connects to the head 50 and has a connection joint 54 for connecting to a gas supply pipe 56. The burner head 50 comprises a plurality of hollow head members 60, including an outer head member 62 and inner head members 64. The illustrated head members 60 have upper 66, side 67 and lower 68 walls. A series of apertures 70 are formed through the lower walls 68 of the head members 60. Preferably, the burner head 50 is substantially sealed except for the apertures 70. As such, the head members 60 collectively provide a gas trap for holding gas under water within a space 72 defined by the head 50.

Pressurized gas from a gas supply, such as commercial gas utility lines or a pressurized gas tank such as a propane tank, is supplied through the throat 52 and into the burner head 50. Such gas collects within the burner head 50, and thus a volume of gas is trapped within the burner head 50 and distributed throughout the head. As additional gas is supplied to the head 50, excess gas flows downwardly out of the apertures 70 and into the surrounding water. Such gas rises to the surface S of the water, where it is ignited to produce the flame effect F.

The arrangement of apertures 70 about the burner head 50 disperses the gas supply so that when the gas reaches the water surface S, the resulting flame F is fairly broad in diameter and has a visually pleasing shape and behavior. Further, the gas dispersion preferably is such that wind gusts and the like will not extinguish the entire flame F. As such, the dispersed gas supply is resistant to flame-out.

Preferably, the head members 60, throat 52 and connecting joint 54 are constructed of a material that is approved for gas distribution under water, such as stainless steel or copper. In the illustrated embodiment, the luminous burner head 50 is constructed of ¾ inch stainless steel tubing having a square cross-section. As shown, the head members 60 are attached by welds 74. Preferably, connecting portions 74 at which the head members are welded together are hollow so that gas is communicated freely between all of the head members 60 and the gas pressure is substantively uniform within the head 50.

In the illustrated embodiment, the apertures 70 are arranged generally centrally in the lower walls 68 of the head members 60 at intervals between about 0.25 and 1.0 inches and more preferably about 0.5 inches. Preferably, the apertures 70 are between about 1/16 and ⅜ inch in diameter. In another embodiment, the apertures 70 are between about 1/16 and ¼ inch. In the illustrated embodiment, the apertures 70 are about 3/32 inch in diameter. Further, the apertures 70 preferably are generally evenly spaced throughout the head members 60.

The burner head 50 preferably is constructed such that the apertures 70 are disposed substantially in a single plane. In addition, when installed under the surface S of a pool of water, the head 50 is disposed so that the aperture plane is substantially horizontal. As such, gas is dispersed generally evenly in the trap space 72 within the head, and flows simultaneously from substantially all of the apertures 70. Since the apertures 70 are spaced about the burner head 50, this arrangement disperses gas bubbles properly so as to create a consistent, broad flame F at the water surface S.

With continued reference to FIGS. 3-5, in the illustrated embodiment, the burner head 50 is substantially square and measures about 18 inches diagonally across the head. It is to be understood that, in further embodiments, larger or smaller burner heads can be provided so as to create different flame sizes and effects. However, it has been found that the size and aperture 70 arrangement described in this embodiment have been particularly effective in creating a consistent flame F that is visually pleasing and remains lit even in relatively windy conditions.

In the illustrated embodiment, the apertures 70 are shown formed through the lower surface of the head members, However, it is to be understood that the apertures 70 may be formed at any location below the upper wall of the head members so that a gas space is provided to properly distribute gas through the burner head 50 and to the apertures 70.

In another embodiment, a luminous burner can employ several smaller burner heads arranged in an array and configured to supply dispersed gas in an arrangement that will create a contiguous flame.

With reference next to FIG. 6, a cut-away portion of an embodiment of the burner head 50 shows the connecting joint and throat 52 of the burner head 50 as it enters the inner head members 64. As discussed above, the connecting joint 54 is configured to connect to and accept gas from a gas supply pipe 56 and to communicate the gas through the throat 52 and into the burner head 50. In the embodiment shown in FIG. 6, the throat 52 enters through the lower wall 68 of the head member 60 and extends upwardly, terminating at a supply opening 78 that is spaced from both the lower wall 68 and the upper wall 66 of the head member 64.

In the illustrated embodiment the supply opening 78 preferably is disposed between about 0.125 inches and 0.625 inches from the bottom wall 68. A gap G is also disposed between the upper wall 66 and the supply opening 78. The gap G preferably is configured so as to not constrict the flow of gas from the supply opening 78 into the head 50. More specifically, the gap distance G between the supply opening 78 and the upper wall 66 is configured so that the area of gas distribution between the supply opening 78 and upper wall 66 is at least equivalent to the cross-sectional area of the throat 52. This area of gas distribution between the supply opening 78 and upper wall 66 is termed an imaginary surface area. In the illustrated embodiment, the throat 52 has an internal diameter of about 0.5 inches, which provides an internal area of about 0.196 square inches (πr²: 3.14*0.25²=0.196 in.²). A gap G of 0.125 inches between the upper wall 60 and the supply opening 78 would produce an imaginary surface area of about 0.196 in.²(2πr*G: 2*3.14*0.25*0.125=0.196 in.²). Thus, since the imaginary surface area between the gas flow supply opening 78 and upper wall 66 is at least equivalent to the throat area, gas flow from the throat 52 into the burner head 50 is not restricted in the illustrated embodiment.

It is to be understood that the minimum gap G will vary depending on other dimensions in the system. In one preferred embodiment wherein the throat 52 has a substantially circular cross section, the gap G preferably is at least about half the radius of the throat 52. It is further to be understood that, in some embodiments, it may be desirable to restrict gas flow into the burner head 50, and a smaller gap G may be in order.

It is anticipated that, especially when the device is not being used, water may enter the burner head 50 through the apertures 70 due to water disturbances caused by wind, splashing, etc. In addition, thermal expansion of the gas and water and condensation due to fluctuating temperatures can enable water to enter the burner head 50. In the illustrated embodiment, the throat 52 acts as a dam to block water within the burner head 50 from spilling into the gas supply line 56 and disrupting the gas supply. Nevertheless, experimentation has shown that, after extended periods of nonuse, water will tend to enter and accumulate in the gas supply line 56.

With reference next to FIG. 7, a schematic illustration is presented of another embodiment of a flame-on-water apparatus 80. The apparatus 80 includes a container 82 having a pool portion 84 and a body portion 86. Preferably the container 80 is disposed on a platform 88 such as a pillar or other architectural structure. A water supply line 90 preferably supplies a flow of water to the container 80, which may include a fountain, spillway, spigot, or the like. The burner head 50 is mounted below the water surface S in the pool portion 84 preferably such that the aperture plane is disposed at a depth “d” about three inches or less below the water surface S. Since utility gas lines are typically pressurized at a relatively low pressure (i.e. about 4 psi), it is preferable to mount the burner 50 generally shallow, such as less than about 5 inches, and more preferably less than about 3 inches below the surface, so that the gas pressure can easily overcome water pressure, and to better control gas bubble dispersion.

The illustrated embodiment includes a gas supply and purge mechanism 94 disposed below the container generally within the support pillar 88. A supply and purge line 100 is connected to the burner head 50 and extends downwardly into the pillar 88. A gas supply line 102 extends from a pressurized gas supply and connects to the supply and purge line 100 at a “T” connection 104. Preferably, the supply line 102 extends upwardly above the “T” 104 and includes a loop portion 106 positioned vertically higher than the T connection 104. A gas supply valve 110 is disposed in the gas line 102 generally downstream of the loop 106. Preferably the gas supply valve 110 is a manually-actuable valve such as a key valve. A portion 112 of the gas supply line 102 between the gas supply valve 110 and the T 104 extends generally downwardly from the valve 110 to the T 104.

The supply and purge line 100 connects to a purge line 120 generally below the gas supply T 104. The illustrated purge line 120 extends through a wall 122 of the pillar 88 and terminates in a purge valve 126. Preferably the purge valve 126 comprises a manually-actuable knurled-nut valve. In another embodiment, the purge line 120 extends generally downwardly from the supply and purge line 100.

An isolation valve 128 is arranged in the supply and purge line 100 between the container 82 and the gas supply connection T 104. The isolation valve 128 is selectively actuable and, when closed, isolates the burner head 50 from both the gas supply line 102 and the purge line 120. Preferably, the combined volume of the burner head 50 and the supply and purge line 100 above the isolation valve 128 is less than the volume of the purge line 120 below the supply T 104. In still another embodiment, the purge line 120 includes a larger-schedule pipe than the supply and purge line 100 and/or the purge line 120 may include a purge tank to increase its volume.

With continued reference to FIG. 7, during normal operation of the device, the purge valve 126 is closed, the gas supply valve 110 is open so as to supply pressurized gas, and the isolation valve 128 is open so that the gas is supplied to the burner head 50. Preferably, when the device is turned off, the user closes both the gas supply valve 110 and the isolation valve 128. As discussed above, due to nonuse or other factors, the burner head 50 and the supply and purge line 100 above the isolation valve 128 may subsequently fill with water from the container. When the device is to be used again, the user first opens the isolation valve 128. Water within the burner will then drain to the purge line 120, and gasses within the purge line 120 and supply and purge line 100 will rise to the burner head 50. Since the volume above the isolation valve 128 is less than the volume in the purge line 120 below the gas supply T 104, the gas supply valve 110 can then be opened, and the purge water will not interfere with gas flow.

To purge the water in the illustrated embodiment, the user preferably first ensures that the isolation 128 and gas supply valves 110 are closed, and then opens the purge valve 126. Water within the system will drain out of the purge valve 126. Once water has been substantially drained, the purge valve 126 is again closed, and the device is ready for use.

If a user mistakenly closes only the gas supply valve 110 when turning off the device and leaves the isolation valve 128 open, the burner head 50 and all piping to the purge valve 126 may eventually become filled with water due to reasons as discussed above. To purge the water in such an instance, the user preferably first closes the isolation valve 128 and then opens the purge valve 126 to drain water from the system. When the water has been drained, the user closes the purge valve 126 and then preferably opens the isolation valve 128 so that the water above the isolation valve 128 enters the purge line 120. The isolation valve 128 is then closed and the purge valve 126 is again opened to drain the water. Such a double purge method should drain substantially all the water from the system.

Although the embodiment illustrated in FIG. 7 shows the container 82 perched on a pillar 88, and valve actuators 130, 136, 138 extending through a generally vertical control panel 140, it is to be understood that other arrangements are contemplated. For example, a luminous burner head 50 may be disposed in an in-ground pool. In such an embodiment, preferably the purge system valves may be actuated via remote actuators. Additionally, such an embodiment preferably would include a drain for the purge line 120 or, in yet another embodiment, additionally include a pump for draining the purge line.

With reference next to FIG. 8, an embodiment of an automated apparatus 150 for generating a flame-on-water effect is illustrated. The illustrated embodiment is structurally very similar to the embodiment illustrated in FIG. 7. However, the isolation 128, gas supply 110, and purge valves 126 each preferably comprise electromechanical valves 128 a, 110 a, 126 a that can be controlled and operated electronically. Preferably, a control panel 152 includes an interface, such as buttons 154, through which a user may control the apparatus 150. User inputs are communicated by electrical communication lines 156 to a controller 160, which analyzes the inputs in light of logic circuitry to control operation of the device. Through electrical communication lines 165 the controller 160 signals respective valves to open and close as needed.

In the illustrated embodiment, sensors 162 are placed in the supply and purge line 100 adjacent the gas supply T 104 and above the isolation valve 128. The sensors 162 are adapted to detect the presence of water at these locations, and communicate the findings back to the controller. In accordance with one embodiment, when a user desires to turn the device on, the controller 160 will analyze the sensor readings and, based on the sensed conditions, determine whether the device needs to be purged prior to startup.

In still another embodiment, a user may set various operating conditions, such as a time limit to automatically turn off the device after a prescribed time period. Additionally, the controller 160 may vary the volume of gas delivered to the burner head 50 according to a random or preset pattern in order to periodically change the behavior of the flame effect.

In still another embodiment, the control panel 152 may interact with and be controllable by a remote controller. Such a remote control unit may perform functions such as turning the apparatus on and off, purging the apparatus, setting time limits, and varying gas flow.

It is to be understood that the gas purge mechanisms disclosed above include presently preferred embodiments. However, it is to be understood that other suitable water purging systems may be used in connection with a burner head in accordance with the present invention.

With reference next to FIGS. 9 and 10, another embodiment of a burner head 170 is constructed of cylindrical tubing 172 connected using standard compression joints 174. The burner head 170 comprises an outer head member 176 connected to inner head members 178. A connection joint 54 and throat 52 are connected to the inner head members 178 in order to supply gas to the head 170. Preferably the inner head members 178 are bent somewhat upwardly so that the supply opening 78 is disposed vertically higher than the upper wall 66 of the outer head member 176. Apertures 70 are provided through a lower surface 68 of the outer head member 176, but not the inner head members 178. Preferably the apertures 70 are spaced between about one quarter to one inch apart, and more preferably about 0.5 inches apart.

With reference next to FIG. 11, a schematic illustration of another embodiment of a supply portion of a burner head 180, including the throat 52, is shown. In this embodiment, the upper wall 66 of the head 180 bulges upwardly adjacent the point where the throat 52 enters the head 180. As such, a gas inlet chamber 182 is provided generally above the rest of the head 180. The bulging portion is considered an inlet chamber upper wall 182. The throat supply opening 78 preferably is disposed at or above the level of the head member upper wall 66. If water were to enter into the burner head 180 during periods of non-use, the water would be further inhibited from entering into the throat 52. Also, condensation that may collect on the inlet chamber 182 upper wall will tend to flow down the sloping chamber wall and away from the throat's supply opening 78.

With reference next to FIG. 12, a schematic diagram of yet another embodiment of a burner head 190 is shown. In this embodiment, a gas inlet chamber 192 is formed above the throat supply opening 78. The gas inlet chamber 192 is disposed generally above the upper wall 66 of the head members 60. The throat 52 also preferably extends upwardly above the upper wall of the head. Further, a splash guard 194 preferably extends downwardly from an edge of the gas inlet chamber 192. In the illustrated embodiment, the splash guard 194 comprises downwardly-directed walls 196 to help block or reduce splashing, etc. that could cause water to enter the throat 52. Preferably, the illustrated downwardly-directed wall 196 extends across the head member between the side walls 67, although only partially from the upper wall 66 to the lower wall 68. It is to be understood that other types of splash guards 194 such as a mesh or gas-permeable membrane, can be used.

With reference next to FIG. 13, another embodiment of a burner head 200 is provided wherein the gas supply tube 202 connects to the burner head 200 through the upper wall 66 of the head 200. In this embodiment, at least a “loop” portion 204 of the supply tube 202 is disposed above the upper wall 66 of the head 200 in what is termed a “Hartford Loop” arrangement. In this embodiment, the aperture plane is still preferably arranged less than about three inches below the surface S of the water.

With reference next to FIGS. 14 and 15, a still further embodiment of a burner head 210 is presented. In the illustrated embodiment the burner head 210 comprises a plurality of head members, including an outer member 212 and four inner head members 214 which connect to the outer head member 212. In the illustrated embodiment, the supply tube 215 extends upwardly to a gas inlet pocket 216 disposed generally above the upper walls 66 of the head members. Supply ports 218 are formed through the upper wall 66 of each of the inner head members 214 so that four separate inlet ports 218 enter the head members 214. Further, the gas flow path extends above the upper wall 66 of the head members 214 and then through the inlet ports 216 into the burner head 210 for delivery to the apertures 70. In another embodiment, the head members are configured so that each supply port supplies a portion of the head members that is sealed off from other portions of the head members, so that the burner head comprises four separate chambers.

With reference next to FIG. 16, still another embodiment of a burner head 220 is provided wherein the burner head 220 comprises an outer head member 222 and four inner head members 224. The gas inlet throat 52 is disposed at the center of the inner members 224, and each of the inner members 224 extends to a corner 226 of the rectangular outer head member 222.

Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or subcombinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow. 

1. A device for delivering unignited gas below a surface of a body of water, comprising: a hollow burner head having an upper surface and a lower surface, a plurality of apertures being formed through the burner member below the upper surface; a gas supply tube having an opening within the head, the opening disposed generally above the level of the apertures.
 2. The device of claim 1, wherein the plurality of apertures are formed through the lower surface.
 3. The device of claim 2, wherein the plurality of apertures are disposed within an aperture plane.
 4. The device of claim 2, wherein the head comprises a tube having a generally rectangular cross-section.
 5. The device of claim 4, wherein the tube has a generally square cross-section.
 6. The device of claim 2, wherein the burner head comprises a tube having a generally circular cross-section.
 7. The device of claim 2, wherein the head comprises a tube having a diameter of about ¾″.
 8. The device of claim 7, wherein the gas supply opening is disposed at least ½″ above the lower surface.
 9. The device of claim 1, wherein the supply tube extends generally upwardly from the lower surface.
 10. The device of claim 9, wherein the supply tube opening is spaced at least ¼″ from the upper surface.
 11. The device of claim 9, wherein the supply tube opening is spaced from the upper surface so that an imaginary surface area between the opening and the upper surface is at least equivalent to the cross-sectional surface area of the supply tube opening.
 12. The device of claim 1, wherein the burner head communicates with a gas inlet chamber having an upper surface, and the gas inlet chamber upper surface is disposed generally higher than the burner head upper surface.
 13. The device of claim 12, wherein the gas supply tube opening is disposed in the gas inlet chamber and at or above the burner head upper surface.
 14. The device of claim 1 additionally comprising a splash guard disposed between the gas supply opening and the plurality of apertures.
 15. The device of claim 14, wherein the splash guard comprises a filter.
 16. The device of claim 14, wherein the splash guard comprises a wall extending downwardly from the upper surface of the burner tube.
 17. The device of claim 1 additionally comprising a control apparatus communicating with the gas supply tube, the control apparatus comprising a water purge device configured to accumulate and selectively purge water within the gas supply tube.
 18. The device of claim 17, wherein the control apparatus additionally comprises a gas supply on/off valve.
 19. The device of claim 1, wherein the apertures have a size between about 1/16″ and ¼″.
 20. A method for supplying unignited gas to a surface of a body of water, comprising: providing a hollow burner having an upper surface and a lower surface, a plurality of apertures being formed through the burner below the upper surface; providing a gas supply tube communicating with the burner, the gas supply tube opening into the burner at a point above the plurality of apertures; and arranging the burner within a body of water so that the apertures are disposed no more than about three inches from a surface of the body of water.
 21. The method of claim 20, wherein the plurality of apertures are arranged substantially within an aperture plane.
 22. The method of claim 21 additionally comprising arranging the burner tube so that the aperture plane is substantially horizontal. 